The present disclosure relates to a memory device to/from which data is written/erased by a change in a conduction type of an amorphous semiconductor layer and, more particularly, to a memory device having an MISFET (Metal Insulator Semiconductor Field Effect Transistor) structure.
With the dramatic spread of small devices for individuals such as information communication devices and, particularly, portable terminals, elements such as memories and logics constructing the devices are demanded to achieve higher performances such as higher integration, higher speed, and lower power. A nonvolatile memory such as a semiconductor flash memory or an FeRAM (Ferroelectric Random Access Memory) is being actively studied and developed for further higher performances.
In recent years, one of nonvolatile memories which are regarded as promising ones is a phase-change memory (refer to, for example, S. Hudgens, et al., “Overview of Phase-Change Chalcogenide Nonvolatile Memory Technology”, MRS BULLETIN NOVEMBER 2004, p. 829). A phase-change memory has a chalcogenide semiconductor layer between two electrodes one of which is connected to a selection diode or a selection transistor, and a part of the chalcogenide semiconductor layer is in contact with one of the electrodes. In the interface with the electrode, the chalcogenide semiconductor layer changes from a crystal state of low electric resistance to an amorphous state of high electric resistance or from the amorphous state to the crystal state by generation of Joule heat. When the crystal state is set as “1” and the amorphous state is set as “0”, by reading the change in the resistance state, “1” and “0” can be discriminated from each other. A resistance value histogram corresponding to the state of “1” and that corresponding to the state of “0” are a resistance separation characteristic which is an important characteristic to increase memory performance.
An amorphous chalcogenide used for a phase-change memory is glass containing a chalcogen element (S, Se, Te) and its representative one is Ge2Sb2Te5 or the like. Glass and an amorphous material are almost equivalent terms. Both of them are solids but do not have the long-range order of a crystal structure unlike liquids. A material having no clear glass-transition point is defined here as an amorphous material.
A phase transition between the amorphous state and the crystal state in the chalcogenide semiconductor layer always accompanies latent heat and is therefore classified as a so-called phase transition of the first kind. The phase transition of the first kind defined here relates to the case where the first order differential (the following expression (1)) of the Gibbs free energy G is discontinuous. In this case, discontinuity occurs in volume or enthalpy. In the expression (1), p denotes pressure of a system, and T denotes absolute temperature of the system. Latent heat necessary for the phase transition of the first kind is equal to discontinuity in enthalpy and, in a state where the pressure or temperature of the system is constant, endothermic reaction or exothermic reaction occurs.
                                                                                          (                                                            ∂                      G                                                              ∂                      P                                                        )                                T                            ⁢                                                          ⁢              OR                                                                          (                                                      ∂                    G                                                        ∂                    T                                                  )                            P                                                          (        1        )            
A chalconide semiconductor is used as a channel layer of a TFT (Thin Film Transistor) in the past. Particularly, a thin film transistor using Te (tellurium) has a relatively good characteristic including hall mobility of about 250 cm2/Vs. However, there are issues such as toxicity of Te, limitation only to a p-type thin film transistor, and magnitude of leak current. With the advent of an amorphous hydrogenated silicon semiconductor, the chalcogenide semiconductor is not used for the channel layer.